Hydraulic classification devices are used extensively throughout the minerals industry to produce differently sized products from a particulate assemblage consisting of a full distribution of particle sizes. Although numerous devices have been developed over the years, a technique currently popular is hindered/fluidized-bed separators. These devices work well for mineral classification if the particle size and density ranges are within acceptable limits.
A great deal of research has been devoted to the study of fluidized-beds and their use in gas/solid contacting and in liquid/solid applications. Studies describing the latter having typically focused on the classification aspects of fluidized-bed separators, although recent work has shown that these devices can also be effectively used for mineral concentration. A hindered-bed separator is a vessel in which water is evenly introduced across the base of the device and rises upward. The separator typically has an aspect ratio of two or more and is equipped with a means of discharging faster settling solids through the bottom of the unit. Rising water and light solids flow over the top of the separator and are collected in a launder. Solids are introduced in the upper portion of the vessel and begin to settle at a rate defined by the particle size and density. The coarser solids settle at a rate that exceeds that of the rising water. A restricted orifice in the base of the separator regulates the discharge of the coarse solids. As a result, a teetering, high-density bed of particles is maintained within the separator. The small interstices within the teeter bed create high interstitial liquid velocities that resist penetration of the finer particles. The fines, therefore, are maintained in the upper portion of the separator and discharge over the top into a collection launder.
It is obvious from the above description that quiescent flow conditions must exist within the separator to maintain a high efficiency. Excessive turbulence and/or changes in flow conditions can result in misplacement of particles. Unfortunately, current teeter-bed separators utilize a feed injection system that discharges directly into the main separation chamber. These systems typically consist of a vertical pipe that terminates approximately one-third of the way into the main separator body. The pipe discharge is usually equipped with a dispersion plate to laterally direct the feed slurry (a mixture of solids and water). This approach creates turbulence within the separator. Additionally, the water that is injected with the feed must also report to the overflow launder. As a result, the rise velocity of the water is substantially increased at the feed injection point. Above the feed point, the liquid rise velocity is the sum of the teeter water and the feed water. This discontinuity often results in a second teeter interface within the separator. In fact, at higher feed rates the volume of water associated with the feed slurry is greater than the volume of teeter water; thus severely affecting unit performance.
Maintenance and reliability are also crucial to long-term separator performance. Conventional teeter-bed separators use a series of lateral pipes located in the base of the separation zone. These pipes are perforated at regular intervals with small diameter holes. Teeter water is injected through these numerous holes over the entire cross-section of the separator. The large water flow rates and small injection hole diameter leave the device susceptible to frequency blockage and plugging due to contaminates in the process water. When several orifices become blocked a dead zone occurs in the fluidization chamber resulting in a loss of performance in this area. It can be seen, therefore, that the conventional hindered-bed separator has inherent design features that limit the capacity and efficiency of the separator. Design modifications are presented that have been demonstrated to provide a higher unit capacity and minimize maintenance aspects of the separator.